What should we do with clothes we bought as gifts for our relatives, knowing that they might wear them without a proper hijab or in front of non-mahram men?

It is not a condition that the abaya be worn over the head. The predominance of conjecture takes the place of certainty in judging the actions of those to whom gifts are given. A mere possibility is not considered, but if it is highly probable that the gifts will be used for unlawful purposes, then do not assist in that. The gift can be a means to advise them and encourage them to do good. The clothes can be dealt with by returning them to the store, using them lawfully, or gifting them to someone who will not use them for unlawful purposes.